The “background” description provided herein is for the purpose of generally presenting the context of the disclosure. Work of the presently named inventors, to the extent it is described in this background section, as well as aspects of the description which may not otherwise qualify as prior art at the time of filing, are neither expressly or impliedly admitted as prior art against the present invention.
MIMO communication techniques are generally deployed for increasing the transmission capacity of a communication system. The MIMO system includes a transmitter and a receiver that communicate via N communication lines (channels) at a transmitting side and N communication lines at a receiving side, wherein N is a integer. For instance, in a wireless communication network, the transmitter may include N transmit antennas and the receiver may include N receiving antennas. Thus, the MIMO communication system enables to secure a communication capacity that is N times larger than that of a single communication line, and as a result yields a greater communication speed as compared to the single communication line.
The technique of MIMO communication used in practise is a single user-MIMO (SU-MIMO). Research efforts are directed towards expanding the SU-MIMO to a multi user-MIMO (MU-MIMO), with a focus of improving the frequency utilization efficiency by using orthogonal channels, a unified operation of multiple cells, and the like.
In MIMO communication systems, the number of antennas deployed at the transmitting side and the receiving side are the same. For instance, in a MIMO communication system that is referred to as N×N MIMO system, there are N antennas included at the transmitting side, and N antennas at the receiving side. A 4×4 MIMO is specified in the Long Term Evolution (LTE) standard of Third generation Paternership Project (3GPP), which serves as a standard for commercially available wireless phones. Further, LTE specifies next generation MIMO systems to be of order 8×8. In order to achieve such standards, a mobile phone terminal would need to be equipped with four to eight antennas. On the other hand, for a mobile phone terminal, there is a requirement and/or desire to reduce the size of the mobile phone casing. Thus, the number of antennas that can be equipped within the mobile terminal is limited due to size constraints. In practice, the maximum number of antennas that can be mounted on the mobile phone terminal is two, and it may be difficult to mount additional antennas as it would increase the design and packaging complexity of the mobile terminal.
Currently, the LTE standard defines a 2×2 MIMO system that is in practical use. For example, in the LTE operated in Japan, the maximum communication speed (theoretically) reaches 37.5 Mbps when 16 QAM signals are used as the subcarriers with a bandwidth of 5 MHz. However, the actual communication speed achieved is approximately 5 Mbps on field tests. It is conceivable that the difference in communication speeds is due to traffic congestion and signal reception quality. The signal reception quality is a characteristic of bit error rate (BER) and signal to noise ratio (SN) ratio. In urban areas such as Tokyo, the SN ratio is usually about 5-10 dB. Even when a 5 dB improvement is achieved by introducing code error correction (by using convolution coding such as Viterbi codes and Reed-Solomon codes), the SN ratio is of the order 10-15 dB. On the other hand, when the 16 QAM modulation is performed by applying a maximum likelihood detection technique in the 2×2 MIMO system, a SN ratio of 15 dB or more is required to secure a BER of 10−3. Thus, in such a communication environment, high-speeds are not realised by merely using a higher order modulation.
In view of the above, a technique capable of reducing the number of antennas at the receiver while maintaining the communication speed by applying a code division multiple access-orthogonal frequency division multiplexing (CDMA-OFDM) technique is described in U.S. provisional patent application No. 61/776,161, and is incorporated herein by reference. A variation of the CDMA-OFDM technique is also described in U.S. provisional patent application No. 61/835,119, and is also incorporated herein by reference.
In the U.S. Provisional applications 61/776,161 and 61/835,119, the characteristic of BER and SN ratio are kept low by adopting a QPSK technique for subcarrier modulation. In doing so, an effective increase in the communication speed in low SN environments is realized. The number of antennas at the receiver may be one or one half (N/2) of the number of transmitter antennas to enable wireless communication at a rate effectively comparable to that of a (4×4) MIMO system. Further, the effective communication speed may be further increased by improving the reception SN ratio and/or introducing diversity in the MIMO system.
Typically, when diversity is implemented, an additional antenna is required. However, in the U.S. provisional applications (61/776,161 and 61/835,119), it was observed that there was no need to increase the number of receiver antennas even when the MIMO system is adopted. Rather, one of the antennas in a terminal was found to be redundant when the MIMO communication technique was applied to a wireless communication of a 2×2 MIMO system. It was anticipated that the communication speed may be further increased when the redundant antenna is utilized for diversity.
To that end, selection diversity and maximum ratio combining diversity are techniques that can be utilized to achieve diverstity in the communication system. Selection diversity is a method for selecting a highest SN ratio antenna from among a plurality of receiver antennas. The maximum ratio combining diversity is a method for combining received signals by matching phases of signals from multiple receiver antennas. The SN ratio may be increased when the maximum ratio combining diversity is implemented.
However, while implementing the above strategies to obtain diversity, the gains of the respective transmission channels may not be suitably adjusted to each other. Specifically, the powers of the received signals over the respective transmission channels may not be equalized and thus such diversity techniques may not be directly applicable.
Accordingly, there is a requirement for a diversity technique for MIMO communication systems, wherein the number of antennas can be reduced while adjusting the gains of the respective transmission channels.